Semiconducting Polymers - American Chemical Society

Chapter 15

Organic and Polymeric Materials for the Fabrication of Thin Film Field-Effect Transistors

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 2, 2018 | https://pubs.acs.org Publication Date: September 16, 1999 | doi: 10.1021/bk-1999-0735.ch015

Zhenan Bao Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, N J 07974

Organic thin-film metal-insulator-semiconductor field-effect transistors (MISFETs) are potentially useful i n low-cost large area flexible displays and low-end data storage such as smart cards. M u c h progress has been made recently in discovering new materials and using low-cost solution-based fabrication processes, such as screen printing techniques. In this paper different semiconducting matrials which have been studied for thin film transistors w i l l be reviewed. Specifically, different aspects which affect the performance of these materials, such as molecular structures, film morphologies, and fabrication conditions, will be discussed.

Organic and polymeric thin-film metal-insulator-semiconductor field-effect transistors ( M I S F E T s ) have received increasing interest recently because o f their potential applications i n low-cost large area flexible displays and low-end data storage such as smart cards (7-2). Organic materials offer numerous advantages for easy processing (e.g. spin-coating, printing, evaporation), good compatibility with a variety o f substrates including flexible plastics, and great opportunities i n structural modifications. Extensive research has been carried out to identify new materials w i t h promising properties, high charge carrier mobility and high current modulation (on/off ratio). Materials with extended π-conjugation, e.g. conjugated oligomers and polymers, have received the most attention. In order for organic M I S F E T s to be useful for liquid crystal displays, the field-effect mobility should be greater than 0.1 2

6

c m A ^ s and the on/off ratio must be higher than 10 . In this paper, we w i l l review recent progress i n material development and low-cost device fabrication techniques for thin film transistors.

244

©1999 American Chemical Society

Hsieh and Wei; Semiconducting Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

245

Experimental

Vacuum deposited devices: The transistor device structure is shown i n Figure l a . A n Η-doped Si was used as substrate, with gold contact that functioned as the gate and an oxide layer o f 3000 Â as the gate dielectric having a capacitance per unit area o f 10 nF/cm . The channel lengths o f the devices were 25, 12, 4, and 1.5 μτη. Semiconducting thin films were prepared by vacuum deposition at a rate o f 4 to 5 Â / s under a pressure o f 2.0 χ 10' Torr, and the thickness o f the resulting films was between 500 to 600 Â . Different substrate temperatures for deposition were obtained by mounting the substrate to a heated copper block controlled by a temperature controller and measured by a thermocouple.


2

6

Printed plastic transistors: The transistor device structures is shown i n Figure l b . A n ITO-coated poly(ethylene terephthalate) film (from Southwall Technologies) is chosen as the plastic substrate. A polyimide ( O P T I M E R A L 3046 from Japan Synthetic Rubber Co.) layer is then printed through a screen mask onto the I T O surface. The screen mask is made o f a stainless steel fabric with 400 mesh count per inch; an emulsion thickness o f about 7.5 u m is used. After being printed, the polyimide dielectric layer is baked at 120° C for an hour. A n organic semiconductor layer consisting o f regioregular poly(3-alkylthiophene)s (from Aldrich Chemical Co.) with different alkyl chain lengths is then put down by spin-coating, casting* or printing using chloroform as the solvent. Finally, the device is completed by printing the drain and source electrodes using a conductive ink (479SS from Acheson Co.) through a screen mask made o f the same fabric and using the same thickness o f emulsion. The drain and source electrodes are two strips 0.5 m m χ 4 m m each, separated by a gap o f 100 μιη, and are about 10 μ ι η thick.

Patterning electrodes using micromolding in capilliaries: Similar steps as described above were used to fabricate all layers except the drain and source electrodes. A elastomeric mold was made by casting and curing polydimethylsiloxane against patterned photoresist. The mold was then brought into conformai contact with a surface and generated a network o f capillary channels; holes machined through the thickness o f the elastomer or molded during the casting and curing step allow access to these channels. Solutions o f polyaniline i n m-cresol, or carbon particles i n ethanol w i c k into the capillary channels when the access holes, which act as reservoirs for the solutions, are filled. Removal o f the elastomer after the solvent evaporates yields an organic conductor patterned i n the geometry o f the mold.

3

The electric characteristics o f these devices were measured under vacuum (10" Torr) unless otherwise specified. The current-voltage characteristics were obtained with a Hewlett-Packard (HP) 4145B analyzer. A t the saturated region, I s (drainsource current) can be described using equation (1), where μ is the field-effect mobility, Wis the channel width, L is the channel length, and C , is the capacitance per D



246

electrodes

organic semiconductor polyimiHe"

ITO(G)

polyester substrate

(b) Figure 1. Device structures o f organic transistors, (a) Bottomcontact device structure; (b) Top-contact device structure Copyright 1997 W i l e y - V C H . (reproduced with permission from reference 33).


247

2

1 / 2

unit area o f the insulating layer ( S i 0 , 3000 Â , C, = 10 n F / c m ). A plot o f I vs. V (gate voltage) can be used to obtain V , the extrapolated threshold voltage, after extrapolation to the V axis. The field-effect mobility can then be calculated from equation (1). X - r a y diffractiograms were obtained i n the reflection geometry using Ni-filtered C u K radiation. Electron microscopy and diffraction was conducted at 2

G

D S

0

G

a

1


100 k V on C-coated films that had been shadowed with Pt at tan" 1/2 to increase contrast.

2

Ιο8 = —μ(ν -νο)

(1)

0

Results and Discussion P-channel materials Vacuum depositedp-channel materials. A number o f conjugated oligomers and metallophthalocyanines have been studeied by different research groups as p-channel semiconducting materials (Table 1). These compunds have limited solubility i n organic solvents and therefore vacuum evaporation has to be used to fabricate their thin films. The highest field-effect mobility has been reported w i t h pentacene ca. 1.5 c m / V s (5), which is i n the same order o f magnitute as amorphous Si (α-Si). Its high performance has been attributed by Laquindanum et al. to the ability o f forming single-crystal-like filmsupon vacuum deposition onto gently heated (about 80 °C) substrates (4). α-Dihexyltetrathienyl (DH-CX-4T) has also been found to form singlecrystal-like films and high field-effect mobility ca. 0.2 c m / V s has been reported b y K a t z et al. (41). We have investigated the transistor behavior of different metallophthalocyanines ( M = Cu,Zn,Pt,Ni,Sn,Fe,H ) since they are commercially available i n large quantity and high purity (10-11). They are also chemically and thermally stable and have been widly used in dye processing, spectral sensitization, chemical sensors, and optical data storage. They were found to function as /7-channel accumulation-mode devices. The charge carrier mobilities o f these devices are strongly dependent on the morphology o f the semiconducting thin films. Highly ordered films are obtained by vacuum deposition at elevated substrate temperatures. Relatively high mobilities (ca. 0.02 c m / V s for Cu-Pc and greater than 10' c m / V s for Z n - P c , SnPc, and H2-Pc) and drain current on/off ratios greater than 10 can be achieved w i t h optimized substrate temperature during deposition ( T ) (Table 2). 2

2

2

2

3

2

4

D

Soluble p-channel materials. To truly realize the advantages (i.e. processability and low-cost) o f organic materials i n device applications, liquid phase processing techniques by spin-coating, casting, or printing are strongly desired. Three methods have been used to fabricate polymer T F T devices form the liquid phase. In the first method, a semiconducting polymer layer is formed directly on the electrodes by electrochemical polymerization, and these electrodes are used subsequently as


248

Table 1. p-Channel organic materials fabricated by vacuum deposition. field-effect

compound

reference

mobility 2


(cm /Vs) 0.002 - 0.02

(5-7)

0.01 - 0.2

(5J-9)

0.003 - 0.02

(10-12)

0.03 - 0.04

(IS)

0.01 - 0.02

(14-15)

0.001 - 0.01

(16)

0.015-0.17

(17)

0.003 - 1.5

(3-4,18-19)

n-2 n = 4-8

C H f i 6

1 3

M = Cu, Sn, Zn, H2

R = H, alkyl



249

Table 2.

Field-effect mobilities for samples deposited at different substrate

temperatures (T ) (reproduced with permission from reference 11). D

M-Pc

T (30°C)

Cu-Pc

6.0 Χ Ι Ο

Sn-Pc

7.3 χ 10"

D

T ( 1 2 5 ° C) D

T ( 2 0 0 ° C) D

3

-4

2.0 χ 10"

2

6.7 χ 10"

5

3.4 xlO"

3

no fieldeffect

H -Pc

1.3 χ 10"

Zn-Pc

2.3 χ 10"

Fe-Pc

7

3

5.6 χ 10"

2.4 χ 10"

3

2.8x10°

5

6.9x10"*

1.1 χ 10"

4

1.5x10^

9.0 χ 10'

6

3.0 χ 10"

5

5.4 χ 10"

3

2.6 χ 10"

4

3.6 xlO"

Pt-Pc

1.5 xlO"

Ni-Pc

7.0 χ 10"

2

5

5

5


250

drain and source electrodes (20-21). In fact, the first organic T F T was fabricated in 1986 by this method using polythiophene as the sefmconducting layer ( μ = 10' 2

5

5

2

c m / V s ) (20). Polypyrrole ( μ = 1.2 χ 10" to 1.77 cm /Vs) and poly(N-alkylpyrrole)s 4

2

( μ = 6.3 χ 10" to 1.74 c m / V s ) have also been prepared by electrochemical synthesis and their T F T properties were studied for these essentially doped films (27).

The

second technique involves the use o f a soluble precursor polymer which can undergo


subsequent chemical reactions to give the desired conjugate oligomer or polymer, such 2

as pentacene ( μ = 0.009 c m / V s ) (22) and poly(thienylene vinylene) ( μ = 0.22 2

cm /Vs) (23).

In these two methods, l o w field-effect mobilities have been reported

except for poly(thienylene vinylene) and polypyrroles, which were doped to achieve high mobility. The l o w mobility i n most o f these materials is probably due to poor ordering and the amorphous nature o f the thin films.

The third technique utilizes

soluble conjugated polymers and they are fabricated by spin-coating, casting, or printing techniques.

W e have studied different conjugated polymers. Examples

including poly(2,5-dialkylpheneylene-co-phenylene)s, poly(2,5-dialkylphenylene-cothiophene)s, poly(2,5-dialkylphenylene vinylene)s and the dialkoxyl derivatives o f the above polymers (24).

4

2

However, very l o w (less than 10" cm /Vs) or no

field-

effect mobilities have been found. M o r e extensive effort has been directed towards soluble polythiophene derivatives since they are widely used as conducing and semiconducting materials (20,25-29).

W e have studied the electrical characteristics o f

field-effect transistors using solution cast regioregular poly(3-hexylthiophene) (P3HT, Figure 3) (30).

It is demonstrated that both high field-effect mobilities (ca. 0.05

2

2

c m / V s i n the accumulation-mode and 0.01 c m / V s in the depletion-mode), and 3

relatively high on/off current ratios (greater than 10 ) can be achieved (Figure 2).

It

was also found that the film quality and field-effect mobility are strongly dependent on the choice o f solvents (30).

4

The field-effect mobility can range from 10" to 10"

2

c m / V s when different solvents are used for film preparation.

2

In addition, treating a

film with ammonia or heating to 100° C under N can increase the on/off ratio without 2

decreasing the mobility (30).

Recently, Sirringhaus et al. have reported

field-effect

2

mobility i n the reange o f 0.05 to 0.1 c m / V s for regioregular poly(3-hexylthiophene) using H M D S treated S i 0 as dielectric layers (42). 2

Another

class o f

liquid

phase

processible

material

is

oligomer-based

compounds which has l o w solubility in organic solvents but enough to form a wellordered thin film with relatively high field-effect mobility. D H - a - 4 T and DH-0C-6T (17,31).

Such materials include 2

The best mobility ca. 0.03 c m / V s has been

obtained with DH-0C-6T from chlorobenzene solution.

Printed plastic transistors The first printed transistor has been demonstrated by Gamier et al. (32).

In

these transistors, however, only the gate electrode and a pair o f drain and source electrodes, were printed separately on each side o f a sheet o f polyester film (1.5 u m thick) w h i c h acts as the dielectric layer. This film with electrodes was then taped to a plastic substrate followed by vacuum deposition o f an organic semiconductor layer o f insoluble dmexyl-a-hexathienylene (DH-0C-6T).

For practical applications, it is

desirable that all the necessary components may be printed i n a continuous process.



251

Ο

-20

-40

-60

-80

-100

Drain-source voltage (V)

Drain-source voltage (V) Figure 2. The current-voltage characteristics o f a F E T w i t h regioregular poly(3-hexylthiophene) semiconducting layer operated i n the accumulation mode (a) and depletion mode (b) at different gate voltages Copyright 1996 American Institute o f Physics (reproduced with permission from reference 30).



252 Therefore, liquid-phase processible organic semiconductors need to be used so that low-cost large area electronics with flexible plastic substrates for display or data storage can be realized by using printing techniques. W i t h the above liquid phase processible materials, we have made the first transistor i n which all the essential components (electrodes, dielectric, and semiconductor) are printed (55). It has been demonstrated that high performance transistors can be made by printing technique on a plastic substrate. These transistors (shown i n Figure l b ) consist o f a polymer dielectric, a semiconducting regioregular poly(3-alkylthiophene), and two silver electrodes, and all o f which have been printed on an ITO-coated plastic substrate. The performance o f these transistors are comparable to those from Si substrate and S i 0 dielectric w i t h lithographically defined A u electrodes (Figure 4). The field effect mobilities are i n the order o f 1 0 ' c m / V s . The smallest channel length can be acieved with the above screen printing technique is about 75 μ ι η . However, much smaller channel length is desired for high current outputs. Recently, we have been able to demonstrate the fabrication o f printed transisotrs with channel length as small as 1 μ ι η using micromolding i n capillaries (34). Different conducting materials, such as polyaniUine and graphite ink, have been sucessfully applied as drain and source electrodes. 2

2

2

Air-stable it-channel matrials Air-stable w-channel semiconducting materials are important components o f pn junction diodes, bipolar transistors, and complementary circuits. The existing «channel materials are either air and moisture-sensitive or have relatively l o w fieldeffect mobilities (Table 3). Recently, we have modified metdlophmalocyanines b y adding strong electron-withdrawing groups such as - C N , - F , and -CI to their outer rings (Figure 5) (55). B y doing so, the L U M O levels o f these molecules are significantly lowered compared to metallophmalocyanines and electron injection and transporting are made possible. Among them, the hexadecafluoro and hexadecachloro metallophthalocyariines were found to function as w-channel semiconductors (55). The best performance has been obtained with Copper hexadecafluoro-phmalocyanine with a field-effect mobility ca. 0.03 cm /Vs (Table 4). W e have found that the high mobilities o f these compounds are the results o f highly ordered films upon vacuum deposition (55). The charge carrier mobilities of these devices are strongly dependent on the morphology o f the semiconducting thin films. Highly ordered films with larger grain sizes, observed by X-ray diffraction and T E M have been obtained by vacuum deposition at elevated substrate temperatures (55). A complementary circuit has been fabricaed using F i C u P c as the w-channel transistor and pentacene as the /^-channel transistor. In addition, all o f the above materials pocess remarkable stability i n air and their transistors can be operated both i n vacuum and i n air. These transistors without any packaging are still functional with high mobilities after stored i n open air for several months. These metallophthalocyanine derivatives are by far the only materials which have been found to have longtime stability i n air with mobilités greater than 10" c m / V s . 2

6

2

2



R

R

RR

head-to-tail (HT)

R

head-to-head (HH)

R

R

R

R

R

=

C

K

6Hl3

regioregular poly(3-hexylthiophene) (P3HT) Figure 3. Chemical structure o f regioregular p o l y ( 3 hexylthiophene) Copyright 1996 American Institute o f Physics (reproduced with permission from reference 30).

0

-10

-20

-30

-40

Drain-source voltage (V) Figure 4. I - V characteristic o f a printed plastic transistor w i t h regioregular poly(3-hexylthiophene) semiconducting layer on polyimide dielectrics coated I T O plastic substrate with printed A g drain and source electrodes.


254


Table 3. Organic η-channel semiconducting materials. compound

field-effect mobility (cm /Vs)

dramatically decreased performance in

reference

0.01-0.08

yes

(36-37)

2

C«/C7

(35)

0.001 - 0.03

0.003

no

(38)

0.003

yes

(38)

3 χ 10"

no

(39)

1.5 χ 10"

yes

(40)

5



255

(CN) CuPc

PyCuPc

8

Figure 5.

Structure o f electron-deficient metallophthalocyanines

(35).

Table 4. Summary of field-effect

2

mobilities (cm /Vs) for different substituted

metallophthalocyanines (reproduced with permission from reference 35). MPc

T (30 °C)

F CuPc

5 χ 10"

F ZnPc

1.7 xlO"

16

16

d

d

3

5

4.3 xlO"

5

5.8 xlO"

3

2.1 xlO"

3

5

a

6

4.5 xlO"

4

1.8 xlO"

5.5 xlO"

d

0.02 4

F FePc

T (215 °C)

1.2 xlO"

4.6 x l O

5

FieCoPc 16

T (125 °C) 0.03

3

Cl FePc

no field-effect

2.7 xlO"

(CN) CuPc

b

b

b

PyCuPc

b

b

b

16

8

a. Compound desorbs at this temperature. b. Compound can not be sublimed.


256

Conclusions Promising transistor performance has been shown with organic and polymeric semiconducting materials with relatively high field-effect mobilities and on/off ratios. The demonstration o f the first plastic transistor by continuous printing techniques is a new step towards printed plastic circuits. Nevertheless, there are still great needs for solution processible p-channel and ^-channel organic materials with mobilités into the range o f 10" cm /Vs. In addition, realiability and lifetime o f organic transistorbased circuits need be investigated i n the near future.


1

2

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(23) (24) (25) (26)


(27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40) (41) (42)

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